Motion Detection for Reflexive Tracking

Abstract

The moving observer who looks in the direction of heading experiences radial optic flow, which is known to elicit horizontal vergence eye movements at short latency, expansion causing convergence and contraction causing divergence: the Radial Flow Vergence Response (RFVR). The moving observer who looks off to one side experiences linear flow, which is known to elicit horizontal version eye movements at short latency: the Ocular Following Response (OFR). Although the RFVR and OFR are very different kinds of eye movement and are sensitive to very different patterns of global motion, they have very similar local spatiotemporal properties. For example, both responses are critically dependent on the Fourier composition of the motion stimuli, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. When the motion stimuli are sine-wave gratings, the two responses share a very similar dependence on the spatial frequency and contrast of those gratings, and even the quantitative details are very similar. When the motion consists of a single step (“two-frame movie”) then a brief inter-stimulus interval results in the reversal of both responses, consistent with the idea that both are mediated by motion detectors that receive a visual input whose temporal impulse response function is strongly biphasic. Further, when confronted with two sine-wave gratings that differ slightly in spatial frequency and have competing motions, both responses show nonlinear dependence on the relative contrasts of those two gratings: when the two sine waves differ in contrast by more than about an octave then the one with the higher contrast completely dominates the responses and the one with lower contrast loses its influence: winner-take-all. It has been suggested that these nonlinear interactions result from mutual inhibition between the low-level mechanisms sensing the motion of the different competing harmonics. Lastly, single unit recordings and local lesions in monkeys strongly suggest that both types of eye movements are mediated by neurons in the MT/MST region of the cerebral cortex that are sensitive to global optic flow. We will argue that these various findings are all consistent with the idea that the RFVR and OFR acquire their different global properties at the level of MT/MST, where the neurons respond to large-field radial and linear optic flow, and their shared local properties from a common earlier stage, the striate cortex, where the neurons respond to the local motion energy.